Metal parts which are exposed to high temperatures often require specially-formulated protective coatings. Aircraft engine parts are but one example. Combustion gas temperatures present in the turbine engine of an aircraft are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000.degree. C.-1100.degree. C. Operation above these temperatures may cause the various turbine components to fail and damage the engine.
The protective coatings, often referred to as thermal barrier coatings or "TBC"s, effectively increase the operating temperatures of the alloys which are used in high-temperature environments. Most of them are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria For a jet engine, the coatings are applied to the surfaces of turbine blades and vanes--usually over an intervening bond layer.
Techniques for depositing thermal barrier coatings like zirconia are known in the art. One method commonly used in the past is plasma-spraying. In this technique, an electric arc is typically used to heat various gasses such as air, nitrogen, or hydrogen to temperatures about 8000.degree. C. or greater. The gasses are expelled from an annulus at high velocity, creating a characteristic flame. Powder material is fed into the flame, and the melted particles are accelerated toward the substrate being coated.
Another technique for depositing thermal barrier coatings is physical vapor deposition (PVD). In one exemplary type of PVD, an ingot of the ceramic material being deposited on the substrate is placed in a chamber which is evacuated. The top end of the ingot is then heated by an intense heat source (from an electron beam or laser, for example), so that it melts and forms a molten pool. A portion of the very hot, molten ceramic evaporates and condenses on the substrate, and a coating is gradually built up as the ingot is raised to replenish the molten pool.
It's clear that there are many advantages to using plasma spray or PVD techniques to deposit thermal barrier coatings. In general, the coatings resulting from either technique are of good quality and durability. Each technique has various advantages over the other. For example, thermal barrier coatings deposited by PVD are normally thinner than those deposited by air plasma spraying, and therefore add less weight to an aircraft engine. They are also especially adherent to smooth, underlying surfaces. On the other hand, plasma-sprayed barrier coatings often provide better insulation than PVD coatings, and their durability in some situations is also exceptional.
However, both plasma spray and PVD exhibit some disadvantages. First, each technique primarily involves line-of-sight deposition. It's therefore very difficult--if not impossible--to coat surfaces in constricted areas. Furthermore, the coating of large parts by PVD is difficult because of size limitations for the required vacuum chambers.
Moreover, repairing TBC's by either technique is difficult. Complete disassembly of the part from attached structures is required for PVD, while partial disassembly of the part may be required for plasma spray. Repairs to the coatings may also require removal of the prior TBC from a large section of the part, along with possible removal of any underlying bond coat (which may also require replacement). Removal of these coatings can be laborious, as can preparation of the part surface for the replacement coatings.
In general, field repairs for TBC's are very difficult with either plasma spray or PVD. Each technique requires large, bulky equipment which is not designed for easy portability. Furthermore, the various factors involved in depositing high quality coatings by either technique may be difficult to maintain in the field.
Thus, it appears that improved methods for providing protective coatings on metal substrates would still be welcome in the art. These techniques would hopefully be especially suitable for the repairs of TBC's in the field, i.e., away from a fixed base which is usually present when large equipment must be used. Moreover, the techniques should be capable of use on small sections of a substrate, without having to strip all of the pre-existing coating from the part. The techniques should also require a minimum of large equipment, since the need for that equipment sometimes lowered productivity in prior art situations.
It's likely that the new coating processes envisioned here may require new coating formulations as well. It's important that these new formulations--once formed into TBC's--possess substantially the same quality as TBC's deposited by plasma spray or PVD. This is especially true when the substrate is a high-performance article like an aircraft engine part.